When an undeflected arc approaches the upper wall of an high intensity discharge (HID) lamp, particularly in small HID lamps, several detrimental effects occur. The envelope overheats, thereby emitting silicon that upsets the lamp chemistry. The silicon combines with the sodium, scandium and other materials, thereby changing the relative amount of iodine (halogen) present in the fill gas. This narrows the arc, causing it to extend higher and closer to the envelope wall. By recycling in this manner, the narrowed arc then tends to cut into the envelope wall causing a voltage rise, heating the lamp further. This process continues until the envelope wall either fails (bubbles out under the fill pressure) or devitrifies (cracks). There is a general need to increase the lamp life, and a particular need to counteract the wall erosion process in small HID lamps.
In vehicle headlamps systems, the lamps usually need to have at least two modes: low beam and high beam. Separate systems may be used, but this approximately doubles the required space, materials and overall cost of the lamp system. It is less expensive to combine them if possible. Vehicle HID systems are costly, although they can provide more, and higher quality light with long system life. A particular advantage to a vehicle HID system is that there is no delicate filament that may fail because of vehicle motion. There is then a general need for single lamps containing optionally selectable, multiple light sources offset from one another, and a particular need for a vehicle HID system with a single bulb providing optical source positions for both the high and low beams.
Increasing lamp efficiency is a continuing goal of lamp makers. In vehicles, headlamps may be the largest single power drain. The electrical system scale is then set to meet headlamp demand. By lowering headlamp demand, the overall scale of the electrical power supply (alternator, regulator, leads, fuses, control boxes, etc.) may be reduced. There is then a need to increase headlamp efficiency, and to reduce vehicle fuel consumption, size, weight and cost.
Magnetic deflection of horizontally operated vehicle HID lamp arcs is known. One system used a bifurcated electrode with a magnetic field system to switch the arc between the upper and lower electrode tip options.